Xylene isomers find wide and varied applications. They are especially valuable as intermediates in chemical syntheses. By way of example, paraxylene (PX) is a feedstock for terephthalic acid, which finds use in the manufacture of synthetic fibers and bottle plastic, metaxylene (MX) is used in the manufacture of dyes, and orthoxylene (OX) is used as a feedstock for phthalic anhydride, which finds use in the manufacture of plasticizers. PX is currently the most valuable of the xylene isomers and although patents related to obtaining (e.g., producing or purifying) PX are too numerous to mention, there is still intensive research in the area.
There are many possible feeds currently used to obtain PX. The majority of paraxylene produced today comes from catalytic reforming, well known in the art, which is primarily fed by naphtha feedstocks. The effluent of the reforming process, known as reformate, is rich in aromatics, particularly toluene and mixed xylenes, and is used as feedstock to aromatics plants. Processes exist to increase the yield of paraxylene over the equilibrium mixture in reformate. These include disproportionation of purified toluene selective methylation of benzene and/or toluene, among others. Other sources described below are also possible feeds to paraxylene production processes.
Steam cracking, or pyrolysis, is the preferred method of producing light olefins (ethylene, propylene, and butenes) from hydrocarbon feedstock. Pyrolysis involves heating of the hydrocarbon feedstock to sufficient temperature for thermal cracking of the molecules to the preferred olefinic products. Also produced in this process is a fraction termed pyrolysis gasoline, steam cracked naphtha (SCN) or pygas, that comprises molecules from C6 to C10+ and is rich in aromatics, particularly benzene and toluene as well as C8, C9 and C10+ aromatics.
Catalytic cracking, or fluid catalytic cracking (FCC), is another well-known process that produces fuels, light olefins, and a similar C6 to C10+ aromatic rich stream, known as “cat naphtha.”
It is known to produce xylenes by the methylation of toluene, for instance methylation of toluene over a catalyst using methanol. U.S. Pat. No. 7,629,498 discloses a process for (a) contacting a pygas feedstock and methylating agent with a catalyst under reaction conditions to produce a product having paraxylenes, wherein the product has higher paraxylene content than the paraxylene content of the feedstock; and (b) separating the paraxylene from the product of step (a). This patent describes steam cracking one or more crude fractions, separating the products into light olefins and pygas, methylating the C8− fraction to form a paraxylene rich product, recovering said paraxylene, and co-recovering light olefins from the PX-rich effluent. This process also mentions the flexibility to extract the aromatics portion from paraffins, naphthenes, and heavy mono- and di-olefins, which cause deactivation and aging of methylation catalysts, to give a purer aromatic feedstock to the reactor.
U.S. Pat. No. 7,301,063 discloses a process to co-produce an aromatic hydrocarbon mixture and liquefied petroleum gas (LPG) from pyrolysis gasoline. The resulting LPG can then be recycled back to the steam cracker which produced the pygas. This process involves contacting the separated pyrolysis gasoline stream, another hydrocarbon feedstock, and hydrogen into at least one reaction area with catalyst, thus converting the mixture into an aromatic fraction rich in benzene, toluene, and xylenes through dealkylation/transalkylation reactions and a fraction rich in LPG through a hydrocracking reaction.
U.S. Pat. No. 7,297,831 discloses a process of preparing aromatic hydrocarbons and liquefied petroleum gas (LPG) from a hydrocarbon mixture, in which a non-aromatic compound in the hydrocarbon feedstock mixture is converted into a gaseous material having a large amount of LPG through hydrocracking, and an aromatic compound therein is converted into an oil component having a large amount of benzene, toluene, and xylene (BTX) through dealkylation and transalkylation.
WO 02/44306 discloses a process for producing aromatic hydrocarbon compounds and liquefied petroleum gas (LPG) from a hydrocarbon feedstock. Aromatic components in the feedstock are converted to BTX-enriched components of liquid phase through hydrodealkylation and/or transalkylation, and non-aromatic components are converted to LPG-enriched gaseous materials through hydrocracking.
U.S. Pat. No. 7,578,929 describes cracking heavy hydrocarbon feedstock containing non-volatile hydrocarbons including mixing the feedstock with a fluid and/or primary dilution steam stream according to a preselected operating parameter of the process.
U.S. Pat. No. 7,563,358 teaches a hydrocarbon conversion process for producing an aromatics product containing one or more BTX species from precursors of the one or more BTX species.
See also U.S. Pat. No. 7,176,339.
The above processes have not been integrated into a single system that offers significant advantages including higher petrochemical yields and lower energy consumption over operation of the processes separately, and wherein benzene and toluene are made “on purpose” from a refinery feedstream in order to provide a more advantageous feed to an alkylation reactor to selectively produce para-xylene (PX).
The present inventors have surprisingly discovered an advantageous system and process, including, in embodiments, the combination of various streams and utilities which provide significant advantages over prior systems.